1. Field of the Invention
This invention relates to battery power supplies, and more particularly to power supplies using metal-air batteries.
2. Description of the Prior Art
The use of metal-air batteries is becoming increasingly prevalent for powering electronic and other devices due to the high energy densities of the batteries. Metal-air batteries require oxygen from a continuous flow of air across the air cathode for optimal battery operation.
Metal-air batteries are typically limited to small sizes and low power outputs because of their sensitivity to humidity in the air. A metal-air battery will fail unless its exposure to water vapor (humidity) in the air is tightly controlled. The equilibrium vapor pressure for the batteries results in an equilibrium relative humidity of about 40%. If the ambient humidity is greater than this equilibrium value, the battery's electrolyte will absorb water from the air through the air cathode and fail due to a condition called flooding. This flooding may cause the battery to leak or to burst.
If the ambient humidity is less than the equilibrium value, water will leave the electrolyte and evaporate through the air cathode. This drying out condition will also cause the battery to fail.
Another problem with a metal-air battery is that oxygen in the air has a tendency to corrode the metal anode by causing the anode to oxidize when the battery is not in use. Also, the presence of air when the battery is not in use causes the battery to self-discharge, resulting in decreased battery output and lifetime.
The battery output is also decreased by the presence of contaminants in the air, including carbon dioxide, cigarette and other smoke, and sulfides. For example, carbon dioxide reacts with the metal hydroxide formed by the reaction between the anode and the electrolyte. The reaction between carbon dioxide and the metal hydroxide forms a metal carbonate compound that interferes with optimal electrolytic action.
Sealing the battery container to keep air from entering it when the battery is not in use will prolong the battery's lifetime. The sealed battery and the air remaining in it will reach the equilibrium humidity of about 40%. The battery will not corrode during non-use, and contaminants cannot be introduced.
However, sealing of the battery causes another problem. A sealed metal-air battery will self-discharge until virtually all of the air trapped in the battery is used up. Without this air, the battery will have only a small initial output power upon startup until an air flow is introduced in the battery. This initial low-power battery output is insufficient to start up most equipment, resulting in a failure of the power supply.